Serves 10 (one 10-inch cake)

Preheat the oven to 400°F.

To Prepare the Pastry:
Place the puff pastry sheets on 2 cookie sheets and prick all over with a fork to prevent them from rising. Bake according to the manufacturer's directions until golden brown (typically, 8 to 10 minutes). Let cool and set aside.

To Prepare the jam:
Purée the quartered strawberries in a blender or food processor. Transfer to a small saucepan and add the corn syrup. In a small bowl, mix the pectin and sugar, add to the saucepan, and set over medium heat. Cook until the mixture appears shiny, 6 to 7 minutes. Remove from the heat, let cool, and set aside.

To Prepare the Mousse:
Place the gelatin in a small metal bowl, add 4 tablespoons of water, and set aside to let soften. Place the cream in a large mixing bowl, whip until soft peaks form, and set aside in the refrigerator.

In a small saucepan, stir together 2 tablespoons of water and the sugar, and set over medium-high heat. Bring to a boil and cook until the mixture reaches about 248°F on a candy thermometer. Place the egg yolks in a mixing bowl and, with an electric beater, start beating on medium speed (or whisk by hand). While whisking, slowly add the sugar syrup and continue to beat until the mixture is thick and cool, 5 to 7 minutes.

Melt the chocolate in a metal bowl set over a small saucepan of simmering water or in a double boiler (or for 35 to 40 seconds in the microwave); keep warm. Transfer the gelatin to the saucepan with simmering water (alternatively, transfer the gelatin and water to a glass bowl and melt in the microwave.) Temper the gelatin by adding some of the beaten egg yolk mixture, then add the remaining egg yolk mixture and whisk to combine. Set aside.

With a spatula, fold a small amount of the refrigerated whipped cream into the warm chocolate, working quickly to prevent lumps from forming. fold in the egg yolk and gelatin mixture, and fold in the remaining whipped cream. Transfer to the refrigerator.

For Assembly:
Lightly butter the bottom and sides of a 10-inch springform pan. Cut a circle slightly less than 10 inches in diameter from a clean piece of cardboard and, using this template, cut two 10-inch circles from the Baked puff pastry. Place 1 of the puff pastry circles, flat side down, on the bottom of the springform pan. Spread a thin layer of the strawberry jam over the puff pastry. Line the inside of the pan with the strawberry halves, with the flat (cut) sides against the rim of the pan and the tips pointing up. Place half of the white chocolate mousse on top of the jam layer. Arrange the sliced strawberries evenly over the mousse, pressing them down into the mousse. Add the remaining mousse on top of the strawberry slices, spreading it evenly to the sides of the pan. Place the second puff pastry circle on top of the mousse with the flat side up, and press it down gently. Refrigerate for at least 3 hours before serving.

To Serve:
Release the clamp on the springform pan and remove the rim. Carefully transfer the Napoleon to a serving plate, dust the top with the powdered sugar, and serve.

Chef's Notes:
This dessert is most easily cut with a serrated knife.

pectin and gelatin sheets are available at specialty baking or gourmet stores. Frozen puff pastry is available at most supermarkets.

For a shortcut, use the best-quality prepared strawberry jam you can find.